branmufn3, it is because coffee grounds--essentially round--won't likely forever wedge between two crossing strands of the steel mesh whereas tea leaves and shavings--like tiny pieces of tracing paper--could easily become stuck in various configurations within the steel mesh. Now, while this might seem negligibly bothersome, their presence in the steel mesh could tarnish the taste of future cups of coffee or tea. However, more seriously, brewing tea in this high-pressure system could potentially block all pathways for the water to escape while brewing, thus making it overflow.
I'm really not sure why they don't reccomend it. I actually use it when one of the kcups leaks grounds in my coffee. I think you could try it. If you did drink tea and the bag breaks you could use it like a sifter too.